Compression ignition engines produce an exhaust gas that contains a variety of pollutants that are the subject of environmental legislation around the world. These pollutants include carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM).
To meet permissible levels of pollutants that may be emitted into the atmosphere set by legislation, exhaust systems for treating the exhaust gas produced by a compression ignition generally contain several emissions control devices. In such exhaust systems, the exhaust gas is usually conducted to a first emissions control device that is able to oxidise carbon monoxide (CO) and the unburned hydrocarbons (HCs) that are present in the gas. The first emissions control device may, for example, be a diesel oxidation catalyst (DOC), a catalysed soot filter (CSF), a NOx storage catalyst (NSC), a passive NOx adsorber (PNA), a diesel exotherm catalyst (DEC), or a cold start concept (CSC™) catalyst.
For NOx emissions, exhaust systems for compression ignition engines may contain a catalyst for the selective catalytic reduction of NOx, such as selective catalytic reduction (SCR) catalyst or selective catalytic reduction filter (SCRF™) catalyst. The selective catalytic reduction (SCR) of NOx primarily occurs by the following three reactions:4NH3+4NO+O2→4N2+6H2O;  (1)4NH3+2NO+2NO2→4N2+6H2O; and  (2)8NH3+6NO→7N2+12H2O.  (3)
The ratio of NO2:NO in the exhaust gas that enters an SCR catalyst or SCRF™ catalyst can affect its performance. In general, SCR catalysts or SCRF™ catalysts show optimum performance when the ratio of NO2:NO is about 1:1. This can be problematic because the exhaust gas produced by a compression ignition engine during normal use typically contains insufficient NO2 (i.e. the ratio of NO2:NO is much lower than 1:1) for optimal performance of the SCR catalyst or the SCRF™ catalyst. To compensate for such low levels of NO2, the first emissions control device often contains a catalytic material that has been formulated to oxidise nitrogen monoxide (NO) to nitrogen dioxide (NO2), thereby increasing the ratio of NO2:NO in the exhaust gas. The SCR catalyst or SCRF™ catalyst is usually arranged downstream of the first emissions control device in an exhaust system, so that the exhaust gas will pass through the first emissions control device before passing through the SCR catalyst or SCRF™ catalyst.
The catalytic material for oxidising NO to NO2 typically comprises platinum (Pt). When the first emissions control device is exposed to relatively high temperatures for a sufficient period of time, low levels of platinum (Pt) may volatilise from the catalytic material and can become trapped on the SCR/SCRF™ catalyst. Such relatively high temperatures may occur during normal use, especially in heavy duty diesel applications, or during filter regeneration, such as when the first emissions control device is a CSF or when there is an upstream diesel particulate filter (DPF). Pt trapped on the SCR catalyst or the SCRF™ catalyst can have a highly detrimental effect on the catalyst's performance because it can oxidise ammonia (NH3). The trapped Pt can consume the NH3 that is intended for the selective catalytic reduction of NO (thereby decreasing NO conversion) and undesirable, secondary emissions may be produced.
The problem of Pt volatilisation is discussed in our publications WO 2013/088133, WO 2013/088132, WO 2013/088128 and WO 2013/050784.